Vertically aligned liquid crystal display device having an optimized viewing angle

ABSTRACT

A method of improving the viewing angle of a vertically-aligned liquid crystal display device is presented. The method involves designing a uniaxial compensation film to provide a retardation value of 200 nm or less for light having a wavelength of about 550 nm. Using this uniaxial compensation film, a display device can be built by obtaining a liquid crystal panel with liquid crystal molecules contained between glass substrates, coupling the uniaxial compensation film to at least one of the glass substrates, and coupling a polarization film and electrodes to the compensation film. Preferably, the uniaxial compensation film has a thickness less than or equal to 50 microns. Where there are multiple compensation films, the total thickness and the total retardation values should be considered.

RELATED APPLICATION

This applications claims priority, under 35 U.S.C. 119, from KoreanPatent Application No. 2002-0040857 filed on Jul. 12, 2002, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display devices.

2. Description of Related Art

A liquid crystal display (“LCD”) device includes upper and lower panelsprovided with field-generating electrodes thereon, a liquid crystallayer interposed therebetween, a pair of a polarizer and an analyzer,compensation films, etc. The LCD generates electric field in the liquidcrystal layer by applying electric voltages to the field-generatingelectrodes and adjusts the intensity of the electric field to controlthe transmittance of light passing through the liquid crystal layer,thereby displaying desired images.

One of the most widely used types of LCD has a common electrode and aplurality of pixel electrodes provided on respective panels and aplurality of thin film transistors (“TFT”) for switching voltagesapplied to the pixel electrodes, which is provided on the panel havingthe pixel electrodes.

LCDs may operate in one of several modes. An LCD operating in avertically-aligned (“VA”) mode contains liquid crystal molecules alignedperpendicular to two panels. VA-mode LCDs are sometimes preferred fortheir high contrast ratio and wide viewing angle.

LCDs often suffer from light leakage, the severity of which increaseswith viewing angle. The light leakage, which causes poor visibility fromthe sides and a narrow viewing angle, is caused by variations in lightpath and in the effective angle made by the polarizer and the analyzerdepending on the viewing directions.

Compensation films are sometimes used to neutralize the effect of thesevariations. However, use of compensation films usually significantlyincreases the cost of the LCD because they are expensive and there is noefficient way to select the compensation film that yields optimalresults. A method of determining the optimal parameters of acompensation film without the costly trial-and-error process is neededin order to allow more LCD applications to take advantage ofcompensation films.

SUMMARY

The invention is a method of improving the viewing angle of avertically-aligned liquid crystal display device and a display devicemade using this method. In more detail, the method involves providingliquid crystal molecules positioned between a first glass substrate anda second glass substrate, and coupling a uniaxial compensation film toat least one of the glass substrates such that the uniaxial compensationfilm provides a retardation value of 200 nm or less for light having awavelength of about 550 nm. When building a display device, a liquidcrystal panel with liquid crystal molecules contained between glasssubstrates is first provided. The liquid crystal molecules arepositioned such that the long axes of the liquid crystal molecules areoriented orthogonal to the glass substrates in the absence of electricalfield. Then, a set of compensation films are coupled to at least one ofthe glass substrates. The set of compensation films includes one or moreuniaxial compensation films and provides a total retardation value ofless than or equal to 200 nm for light having a wavelength of about 550nm. A polarization film is coupled to the set of compensation films, andcoupling electrodes are coupled to the liquid crystal panel.

As stated above, the invention also includes a display device builtusing the above method. More specifically, the invention includes adisplay device including a liquid crystal layer disposed between glasssubstrates so that long axes of liquid crystal molecules are orientedorthogonal to the glass substrates, a set of compensation films coupledto at least one of the glass substrates, wherein the set of compensationfilms are selected based on having a total retardation value less thanor equal to 200 nm for light having a wavelength of about 550 nm, apolarization film coupled to the set of compensation films, and a firstelectrode and a second electrode coupled to the glass substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the present invention will become moreapparent by describing preferred embodiments thereof in detail withreference to the accompanying drawings in which:

FIG. 1 is a sectional view of an LCD according to an embodiment of thepresent invention;

FIG. 2 is a layout view of a TFT array panel of an LCD according to anembodiment of the present invention;

FIG. 3 is a sectional view of the TFT array panel shown in FIG. 2 takenalong the line III-III′;

FIG. 4 is a layout view of a color filter array panel of an LCDaccording to an embodiment of the present invention;

FIG. 5 is a sectional view of an LCD according to another embodiment ofthe present invention;

FIG. 6 is a sectional view of a TFT array panel of an LCD according toanother embodiment of the present invention;

FIG. 7 is a sectional view of an LCD according to another embodiment ofthe present invention;

FIG. 8 is a layout view of a TFT array panel of an LCD according toanother embodiment of the present invention;

FIG. 9 is a sectional view of the TFT array panel shown FIG. 8 takenalong the line IX-IX′;

FIG. 10 is a graph showing reflectance as function of applied voltagefor a transflective LCD with and without uniaxial (C-plate) compensationfilms;

FIG. 11 is a graph showing transmittance as function of applied voltagefor a transflective LCD with and without uniaxial (C-plate) compensationfilms;

FIGS. 12A to 12F are graphs showing isocontrast curves of a reflectivetype LCD without and with one C-plate attached to the upper panel;

FIGS. 13A to 13F are graphs showing isocontrast curves of a transmissivetype LCD without and with one C-plate attached to the upper panel;

FIGS. 14A to 14E are graphs showing isocontrast curves of a reflectivetype LCD with C-plates attached to both upper and lower panels; and

FIGS. 15A to 15E are graphs showing isocontrast curves of a transmissivetype LCD with C-plates attached to both upper and lower panels.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein.

In the drawings, the thickness of layers and regions are exaggerated forclarity. Like numerals refer to like elements throughout. It will beunderstood that when an element such as a layer, film, region, substrateor panel is referred to as being “on” another element, it can bedirectly on the other element or on one or more intervening elements. Incontrast, when an element is referred to as being “directly on” anotherelement, there are no intervening elements present.

Then, LCDs according to embodiments of the present invention will bedescribed in detail with reference to the drawings.

FIG. 1 is a sectional view of a transmissive type LCD according to anembodiment of the present invention.

An LCD according to this embodiment includes a TFT array panel 1 and acolor filter array panel 2 facing each other, and a liquid crystal layer3 interposed between the two panels 1 and 2. The LCD also includes firstand second polarization films 12 and 22 having nonparallel polarizationaxes, and first and second protective films 13 and 23 preferably made ofTAC (triacetate cellulose) films and attached on the first and thesecond polarization films 12 and 22 for protecting the polarizationfilms 12 and 22, respectively. The LCD further includes a first uniaxial(C-plate) compensation film 14 inserted between the TFT array panel 1and the first protective film 13, and a second uniaxial compensationfilm 24 inserted between the color filter array panel 2 and the secondprotective film 23.

The LCD is in a vertically-aligned (VA) mode. That is, the liquidcrystal layer 3 of the LCD includes liquid crystal molecules aligned tomake their long axes substantially perpendicular to the two panels 1 and2.

The first and the second protective films 13 and 23 generate slightretardation. In addition, the uniaxial compensation films 14 and 24 havenegativity and generate retardation in a range between about 0 nm andabout 200 nm for the light having a wavelength of 550 nm. Here, theuniaxiality means that nx=ny≠nz and the negativity means that nx=ny>nz,where nx, ny and nz denote the refractive indices of x, y and zdirections, respectively.

The first uniaxial compensation film 14 may be omitted.

Now, a TFT array panel and a color filter array panel of an LCDaccording to embodiments are described in more detail.

FIG. 2 is a layout view of a TFT array panel for an LCD according to anembodiment of the present invention, and FIG. 3 is a sectional view ofthe TFT array panel shown in FIG. 2 taken along the line III-III′.

As shown in FIGS. 2 and 3, a gate wire 121, 123 and 125 preferably madeof a metal having low resistivity such as aluminum, silver, etc. isformed on a transparent insulating substrate 110. The gate wire 121, 123and 125 includes a plurality of gate lines 121 extending in a transversedirection and a plurality of gate electrodes 123 connected to the gatelines 121. An end portion 125 of each gate line 121 is widened forconnection with an external circuit.

A gate insulating layer 140 is formed on the entire surface of thesubstrate including the gate wire 121, 123 and 125.

A plurality of semiconductor stripes 151, 153 and 159 preferably made ofamorphous silicon are formed on the gate insulating layer 140, and aplurality of ohmic contacts 161, 162, 163 and 165 preferably made ofamorphous silicon heavily doped with n-type impurity are formed on thesemiconductor stripes 151, 153 and 159.

A data wire 171, 173, 175, 177 and 179 preferably made of a metal havinglow resistivity such as aluminum, silver, etc. is formed on the ohmiccontacts 161, 162, 163 and 165 and the gate insulating layer 140.

The data wire 171, 173, 175, 177 and 179 includes a plurality of datalines 171 intersecting the gate lines 121 to define a plurality of pixelareas, a plurality of source electrodes 173 which are branches of thedata lines 171 and connected to the ohmic contacts 163, a plurality ofdrain electrode 175 separated from the source electrodes 173 and formedon the ohmic contacts 165 opposite to the source electrodes 173 withrespect to the gate electrodes 123, and a plurality of storageelectrodes 177 overlapping the gate lines 121 to form storagecapacitors. An end portion 179 of each data line 171 is widened forconnection with an external circuit.

A passivation layer 180 is formed on the data wire 171, 173, 175, 177and 179. The passivation layer 180 has a plurality of first contactholes 181 exposing the drain electrodes 175, a plurality of secondcontact holes 182 exposing the end portions 125 of the gate lines, aplurality of third contact holes 183 exposing the end portions 179 ofthe data lines 171, and a plurality of fourth contact holes 184 exposingthe storage electrodes 177.

A plurality of pixel electrodes 190 and a plurality of contactassistants 95 and 97 are formed on the passivation layer 180. The pixelelectrodes 190 are connected to the drain electrodes 175 and the storageelectrodes 177 via the first and the fourth contact holes 181 and 184,respectively, and the contact assistants 95 and 97 are connected to theexposed end portions 125 of the gate lines 121 and the exposed endportions 179 of the data lines 171 via the second and the third contactholes 182 and 183, respectively. The pixel electrodes 190 and thecontact assistants 95 and 97 are preferably made of transparent materialsuch as ITO (indium tin oxide) or IZO (indium zinc oxide).

FIG. 4 is a layout view of a color filter array panel of an LCDaccording to an embodiment of the present invention.

A black matrix 220 is formed on an insulating substrate 210, a pluralityof color filters 230 are formed on the black matrix 220, and a commonelectrode 270 is formed on the color filters 230. The common electrode270 is preferably made of a transparent conductive material such as ITOor IZO.

FIG. 5 is a sectional view of a reflective type LCD without separatelight source according to another embodiment of the present invention.

An LCD according to this embodiment includes a TFT array panel 1 and acolor filter array panel 2 facing each other, and a liquid crystal layer3 interposed between the two panels 1 and 2. The LCD further includes apolarization film 22 and a protective film 23 attached on thepolarization film 22 for protecting the polarization film 22. The LCDalso includes a uniaxial compensation film 24 and a reverse dispersionphase difference film 25 inserted between the color filter array panel 2and the protective film 23.

The LCD is in a VA mode. The protective film 23 generates slightretardation, and the uniaxial compensation film 24 has negativity andgenerates retardation ranging 0 nm to 200 nm for the light with 550 nmwavelength.

FIG. 6 is a sectional view of a TFT array panel of an LCD according toanother embodiment of the present invention.

Referring to FIG. 6, a gate wire 121, 123 and 125, a gate insulatinglayer 140, a plurality of semiconductor stripes 151 and 153, a pluralityof ohmic contacts 161, 162, 163 and 165, a data wire 171, 173, 175, 177and 179, a passivation layer 180, a plurality of pixel electrodes 190,and a plurality of contact assistants 95 and 97 are formed on asubstrate 110.

The surface of the passivation layer 180 has embossment includingprominences/protrusions and depressions, and the pixel electrodes 190are preferably made of a metal having good reflectance such as aluminum.

FIG. 7 is a sectional view of a transflective LCD according to anotherembodiment of the present invention.

An LCD according to this embodiment includes a TFT array panel 1 and acolor filter array panel 2 facing each other, and a liquid crystal layer3 interposed between the two panels 1 and 2. The LCD also includes apair of first and second polarization films 12 and 22, and a pair offirst and second protective films 13 and 23 attached on the polarizationfilms 12 and 22, respectively. The LCD further includes a first uniaxial(C-plate) compensation film 14 and a first reverse dispersion phasedifference film 15 inserted between the TFT array panel 1 and the firstprotective film 13, and a second uniaxial (C-plate) compensation film 24and a second reverse dispersion phase difference film 25 insertedbetween the color filter array panel 2 and the second protective film23.

The LCD is in a VA mode. The first and the second protective films 13and 23 generate slight retardation, and the first and the seconduniaxial compensation films 14 and 24 have negativity and generateretardation in a range from 0 nm to 200 nm for the light with 550 nmwavelength. The first uniaxial compensation film 14 may be omitted.

FIG. 8 is a layout view of a TFT array panel of an LCD according to anembodiment of the present invention, and FIG. 9 is a sectional view ofthe TFT array panel shown in FIG. 8 taken along the line IX-IX′.

Referring to FIG. 8, a gate wire 121, 123 and 125, a gate insulatinglayer 140, a plurality of semiconductor stripes 151 and 153, a pluralityof ohmic contacts 161, 162, 163 and 165, a data wire 171, 173, 175, 177and 179, and a passivation layer 801 are formed on a substrate 110.

A plurality of transparent electrodes 90 and a plurality of contactassistants 95 and 97 preferably made of ITO or IZO are formed on thepassivation layer 801. An interlayer insulating layer 802 having anembossed surface is formed on the transparent electrodes 90. A pluralityof reflecting electrodes 80 are formed on the interlayer insulatinglayer 802, and each reflecting electrodes 80 has a window 82 for lighttransmission.

Various characteristics of various types of LCDs with various types ofcompensation films were measured.

The LCDs used for the measurement have conditions shown in TABLE 1 andTABLE 2.

TABLE 1 Mode VA Dopant natural pitch of 67 microns Twist Angle 90degrees Pretilt Angle 89 degrees K11 13.0 pN K22 5.1 pN K33 14.7 pN ε∥3.6 ε⊥ 7.4 Cell Gap 2.89 microns

Here, K11, K22 and K33 are elastic coefficients of spreading, twistingand bending measured in pico-newton (pN) and ε_(∥) and ε⊥ arepermittivity parallel to and perpendicular to the director,respectively.

TABLE 2 Δnd for Thickness 550 nm n∞ A(nm⁻ ²) (microns) wavelength LiquidVA ne 1.5369 7651.0 2.89 240 nm Crystal no 1.4607 5569.0 Reverse ne1.5934 −268.8 52.14 142.86 nm dispersionλ/ no 1.59 0 4 plate TAC nx nynz 80 — 1.4800 1.4798 1.4791 C-Plate nx ny nz 20 80 nm 1.504  1.5041.500

Here, ne is the refractive index parallel to the director (i.e. forextraordinary ray) and no is the refractive index perpendicular to thedirector (i.e. for ordinary ray), while Δn=ne−no. In addition, thedispersion relation is given by:

${{n(\lambda)} = {n_{\infty} + \frac{A}{\lambda^{2}}}},$

where n_(∞) is the refractive index for infinite wavelength and A is aconstant.

FIGS. 10 and 11 are graphs respectively showing reflectance andtransmittance as function of applied voltage for a transflective LCDwith and without uniaxial (C-plate) compensation films.

The curves show that the presence of the uniaxial compensation filmshardly affects the reflectance and the transmittance of the LCD.

FIGS. 12A to 12F are graphs showing isocontrast curves of a reflectivetype LCD without and with one C-plate attached to the upper panel. FIGS.12A to 12F show the isocontrast curves for the cases 2 to 7 in the TABLE3, respectively.

TABLE 3 Reflective Type Number/ Viewing Angle Areal Thickness(um)/Reflectance Front (up/down/left/right) Isocontrast Case Mode Δnd of Cplate (%) view CR CR 2:1 Ratio (CR 10:1) 1 TN None 11.7 19.9 47/34/80/660.861 2 VA None 16.9 26.0 68/68/51/51 0.757 3 VA  One/20/80 nm 16.9 25.880/80/79/79 1 4 VA One/40/160 nm 16.9 24.0 55/55/68/68 1.324 5 VAOne/60/240 nm 16.9 22.1 42/42/50/50 0.987 6 VA One/80/320 nm 16.9 26.635/35/42/42 0.723 7 VA One/100/400 nm  16.9 26.6 31/31/36/36 0.603

FIGS. 13A to 13F are graphs showing isocontrast curves of a transmissivetype LCD without and with one C-plate attached to the upper panel. FIGS.13A to 13F show the isocontrast curves for the cases 2 to 7 in TABLE 4,respectively.

TABLE 4 Transmissive type Number/ Viewing Angle Areal Thickness(um)/Transmittance Front (up/down/left/right) Isocontrast Case Mode Δnd of Cplate (%) View CR CR 2:1 Ratio (CR 10:1) 1 TN None 7.4 378.4 59/59/80/801.065 2 VA None 13.0 881.6 80/47/80/80 1.404 3 VA  One/20/80 nm 13.0880.3 80/40/80/79 1.55 4 VA One/40/160 nm 13.0 881.9 60/34/70/63 1.410 5VA One/60/240 nm 13.0 880.7 50/31/57/54 1.177 6 VA One/80/320 nm 13.0881.0 44/30/50/49 0.925 7 VA One/100/400 nm  13.0 882.0 39/27/45/440.797

FIGS. 14A to 14E are graphs showing isocontrast curves of a reflectivetype LCD with two C-plates respectively attached to upper and lowerpanels. FIGS. 14A to 14E show the isocontrast curves for Cases 8 to 12,respectively.

TABLE 5 Reflective type Viewing Angle Areal Thickness(um)/ ReflectivityFront (up/down/left/right) Isocontrast Case Mode Δnd of C plate (%) viewCR CR 2:1 Ratio (CR 10:1) 8 VA  10/40 nm × 2 16.9 23.2 75/78/62/62 0.8619 VA  20/80 nm × 2 16.9 25.8 80/80/79/79 1 10 VA 30/120 nm × 2 16.9 24.369/69/80/80 1.209 11 VA 40/160 nm × 2 16.9 24.0 55/55/68/68 1.324 12 VA50/200 nm × 2 16.9 24.3 47/47/58/58 1.242

FIGS. 15A to 15E are graphs showing isocontrast curves of a transmissivetype LCD with two C-plates respective attached to upper and lowerpanels. FIGS. 15A to 15E show the isocontrast curves for Cases 8 to 12in TABLE 6, respectively.

TABLE 6 Transmissive Type Viewing Angle Areal Thickness(um)/Transmittance Front (up/down/left/right) Isocontrast Case Mode Δnd of Cplate (%) View CR CR 2:1 Ratio (CR 10:1) 8 VA  10/40 nm × 2 13.0 881.080/39/80/78 1.565 9 VA  20/80 nm × 2 13.0 881.8 59/34/75/63 1.430 10 VA30/120 nm × 2 13.0 881.2 49/31/58/54 1.215 11 VA 40/160 nm × 2 13.0881.6 43/29/51/48 0.958 12 VA 50/200 nm × 2 13.0 882.1 38/27/46/44 0.817

In TABLES 3 to 6, the areal isocontrast ratio is an isocontrast area forthe contrast ratio of 10:1 divided by that in Case 3 of the reflectivetype LCD. White and black voltages in VA mode are 3.5V and 1.8V for thereflective type and 4.5V and 1.8V for transmissive type, respectively.The abbreviation “CR” stands for contrast ratio.

The ratios such as 2:1, 5:1, 10:1, 20:1 and 22:1 in the legends of FIGS.12A to 15E indicate contrast ratios and the values (for example,68/68/51/51 in FIG. 12A) at the bottom of FIGS. 12A to 15E indicateupper/lower/left/right side viewing angles giving the contrast ratio of2:1.

The measurement values of TABLES 3 to 6 shown in FIGS. 12A to 12F, 13Ato 13F, and 14A to 15E can be summarized as follows.

Without uniaxial compensation film (C-plate), the transmittance, thereflectance, the contrast ration at the front view, and the viewingangle of the VA mode are superior to those of the TN mode.

Compared with Case 2, which does not include a uniaxial compensationfilm, Cases 3 and 4 of the VA mode show both improved viewing angle andimproved isocontrast curve, and Cases 3 and 4 of the transmissive modeshow improved isocontrast curves. In contrast, Cases 5, 6 and 7, each ofwhich includes a compensation film causing a retardation greater than160 nm, show deteriorated viewing angle and deteriorated isocontrastcurves. Data indicates that a uniaxial compensation film providing aretardation value larger than 160 nm has a detrimental effect on the LCDdevice.

With uniaxial compensation films attached to both upper and lowerpanels, the isocontrast curves for the transmissive-type LCDs areimproved until the sum of the retardation values of the two compensationfilms equals to 160 nm. When the combined retardation value exceeds 160nm, both the isocontrast curves and the viewing angles become worse. Forthe reflective-type LCDs, only one of the two compensation filmscontributes to the total retardation since light is not transmittedthrough both of films. Therefore, the actual retardation values of thecompensation films for the cases 8 to 11 are equal to or smaller than160 nm. This explains why case 12 for the reflective-type LCD showsimproved isocontrast curves in spite of having a retardation value of200 nm. For the transmissive-type LCD, although there is no experimentalexample for the case of 200 nm retardation, the results shown in TABLES3 to 6 suggest that the isocontrast curves will be improved whereretardation values are equal to or less than 200 nm.

The above-described experimental results show a correlation between thetotal thickness of the uniaxial compensation film(s) and the viewingangle and/or the contrast ratio. It appears that the total thickness ofthe uniaxial compensation film(s) affects the viewing angle and/or thecontrast ratio more than the number or the physical arrangement of theuniaxial compensation film(s).

Of the cases above, Case 3 has optimal characteristics both for thetransmissive type and the reflective type LCDs. Although some cases showbetter contrast ratio at the front view than Case 3, the difference issmall. Overall, Case 3 resulted in a better viewing angle than the othercases. Therefore, it can be said that Case 3 is optimized.

In conclusion, the total retardation of the uniaxial compensationfilm(s) equal to or under 200 nm improves the isocontrast curve and/orthe viewing angle. This improvement is irrelevant to the number ofuniaxial compensation films used and the type (reflective or thetransmissive) of LCD.

According to the present invention, uniaxial compensation film(s)generating a predetermined retardation is used to improve the viewingangle of the LCD.

1. A method of building a display device, the method comprising:providing a liquid crystal panel including liquid crystal moleculescontained between glass substrates, wherein long axes of the liquidcrystal molecules are oriented orthogonal to the glass substrates inabsence of electrical field; coupling a set of compensation films to atleast one of the glass substrates, wherein the set of compensation filmsincludes one or more uniaxial compensation films and provides a totalretardation value less than or equal to 200 nm for light having awavelength of about 550 nm; coupling a polarization film to the set ofcompensation films; and coupling electrodes to the liquid crystal panel.2-23. (canceled)